Double-layer coating for high-resolution low-temperature scanning electron microscopy

ESEM Methodology for the Study of Ice Samples at Environmentally Relevant Subzero Temperatures: "Subzero ESEM"

Frozen aqueous solutions are an important subject of study in numerous scientific branches including the pharmaceutical and food industry, atmospheric chemistry, biology, and medicine. Here, we present an advanced environmental scanning electron microscope methodology for research of ice samples at environmentally relevant subzero temperatures, thus under conditions in which it is extremely challenging to maintain the thermodynamic equilibrium of the specimen. The methodology opens possibilities to observe intact ice samples at close to natural conditions. Based on the results of ANSYS software simulations of the surface temperature of a frozen sample, and knowledge of the partial pressure of water vapor in the gas mixture near the sample, we monitored static ice samples over several minutes. We also discuss possible artifacts that can arise from unwanted surface ice formation on, or ice sublimation from, the sample, as a consequence of shifting conditions away from thermodynamic equilibrium in the specimen chamber. To demonstrate the applicability of the methodology, we characterized how the true morphology of ice spheres containing salt changed upon aging and the morphology of ice spheres containing bovine serum albumin. After combining static observations with the dynamic process of ice sublimation from the sample, we can attain images with nanometer resolution.

Studying the Supramolecular Organization of Photosynthetic Membranes within Freeze-fractured Leaf Tissues by Cryo-scanning Electron Microscopy

Cryo-scanning electron microscopy (SEM) of freeze-fractured samples allows investigation of biological structures at near native conditions. Here, we describe a technique for studying the supramolecular organization of photosynthetic (thylakoid) membranes within leaf samples. This is achieved by high-pressure freezing of leaf tissues, freeze-fracturing, double-layer coating and finally cryo-SEM imaging. Use of the double-layer coating method allows acquiring high magnification (>100,000X) images with minimal beam damage to the frozen-hydrated samples as well as minimal charging effects. Using the described procedures we investigated the alterations in supramolecular distribution of photosystem and light-harvesting antenna protein complexes that take place during dehydration of the resurrection plant Craterostigma pumilum, in situ.

Photoprotection conferred by changes in photosynthetic protein levels and organization during dehydration of a homoiochlorophyllous resurrection plant

During desiccation, homoiochlorophyllous resurrection plants retain most of their photosynthetic apparatus, allowing them to resume photosynthetic activity quickly upon water availability. These plants rely on various mechanisms to prevent the formation of reactive oxygen species and/or protect their tissues from the damage they inflict. In this work, we addressed the issue of how homoiochlorophyllous resurrection plants deal with the problem of excessive excitation/electron pressures during dehydration using Craterostigma pumilum as a model plant. To investigate the alterations in the supramolecular organization of photosynthetic protein complexes, we examined cryoimmobilized, freeze-fractured leaf tissues using (cryo)scanning electron microscopy. These examinations revealed rearrangements of photosystem II (PSII) complexes, including a lowered density during moderate dehydration, consistent with a lower level of PSII proteins, as shown by biochemical analyses. The latter also showed a considerable decrease in the level of cytochrome f early during dehydration, suggesting that initial regulation of the inhibition of electron transport is achieved via the cytochrome b6f complex. Upon further dehydration, PSII complexes are observed to arrange into rows and semicrystalline arrays, which correlates with the significant accumulation of sucrose and the appearance of inverted hexagonal lipid phases within the membranes. As opposed to PSII and cytochrome f, the light-harvesting antenna complexes of PSII remain stable throughout the course of dehydration. Altogether, these results, along with photosynthetic activity measurements, suggest that the protection of retained photosynthetic components is achieved, at least in part, via the structural rearrangements of PSII and (likely) light-harvesting antenna complexes into a photochemically quenched state.

Electron tomography and cryo-SEM characterization reveals novel ultrastructural features of host-parasite interaction during Chlamydia abortus infection

Chlamydia (C.) abortus is a widely spread pathogen among ruminants that can be transmitted to women during pregnancy leading to severe systemic infection with consecutive abortion. As a member of the Chlamydiaceae, C. abortus shares the characteristic feature of an obligate intracellular biphasic developmental cycle with two morphological forms including elementary bodies (EBs) and reticulate bodies (RBs). In contrast to other chlamydial species, C. abortus ultrastructure has not been investigated yet. To do so, samples were fixed by high-pressure freezing and processed by different electron microscopic methods. Freeze-substituted samples were analysed by transmission electron microscopy, scanning transmission electron microscopical tomography and immuno-electron microscopy, and freeze-fractured samples were analysed by cryo-scanning electron microscopy. Here, we present three ultrastructural features of C. abortus that have not been reported up to now. Firstly, the morphological evidence that C. abortus is equipped with the type three secretion system. Secondly, the accumulation and even coating of whole inclusion bodies by membrane complexes consisting of multiple closely adjacent membranes which seems to be a C. abortus specific feature. Thirdly, the formation of small vesicles in the periplasmic space of RBs in the second half of the developmental cycle. Concerning the time point of their formation and the fact that they harbour chlamydial components, these vesicles might be morphological correlates of an intermediate step during the process of redifferentiation of RBs into EBs. As this feature has also been shown for C. trachomatis and C. pneumoniae, it might be a common characteristic of the family of Chlamydiaceae.

Three-dimensional imaging of adherent cells using FIB/SEM and STEM

In this chapter we describe three different approaches for three-dimensional imaging of electron microscopic samples: serial sectioning transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) tomography, and focused ion beam/scanning electron microscopy (FIB/SEM) tomography. With these methods, relatively large volumes of resin-embedded biological structures can be analyzed at resolutions of a few nm within a reasonable expenditure of time. The traditional method is serial sectioning and imaging the same area in all sections. Another method is TEM tomography that involves tilting a section in the electron beam and then reconstruction of the volume by back projection of the images. When the scanning transmission (STEM) mode is used, thicker sections (up to 1 μm) can be analyzed. The third approach presented here is focused ion beam/scanning electron microscopy (FIB/SEM) tomography, in which a sample is repeatedly milled with a focused ion beam (FIB) and each newly produced block face is imaged with the scanning electron microscope (SEM). This process can be repeated ad libitum in arbitrary small increments allowing 3D analysis of relatively large volumes such as eukaryotic cells. We show that resolution of this approach is considerably improved when the secondary electron signal is used. However, the most important prerequisite for three-dimensional imaging is good specimen preparation. For all three imaging methods, cryo-fixed (high-pressure frozen) and freeze-substituted samples have been used.

Freeze fracture approach to directly visualize wetting transitions on nanopatterned superhydrophobic silicon surfaces: more than a proof of principle

Freeze fracturing is applied to make the wetting behavior of artificially nanopatterned Si surfaces directly visible. For this purpose, almost hexagonally arranged nanopillars of fixed areal density (127 μm(-2)) and diameters (35 nm) but varying heights (40-150 nm) were fabricated on silicon. Measurement of contact angles (CAs) including hysteresis allowed to distinguish between the Wenzel (W) and the Cassie-Baxter (CB) states with droplets completely wetting the pillars or residing on top of them, respectively. Providing additional depth contrast by evaporating the ice replica with thin carbon and (typically 3 nm) platinum layers under 45° allowed resolving 3D features of 5 nm within the ice replica. In this way, laterally sharp transitions from CB- to W-states could be revealed, indicating the formation of zero-curvature water surfaces even on the nanoscale.

High-pressure freezing for scanning transmission electron tomography analysis of cellular organelles

Using an electron microscope's scanning transmission mode (STEM) for collection of tomographic datasets is advantageous compared to bright field transmission electron microscopic (TEM). For image formation, inelastic scattering does not cause chromatic aberration, since in STEM mode no image forming lenses are used after the beam has passed the sample, in contrast to regular TEM. Therefore, thicker samples can be imaged. It has been experimentally demonstrated that STEM is superior to TEM and energy filtered TEM for tomography of samples as thick as 1 μm. Even when using the best electron microscope, adequate sample preparation is the key for interpretable results. We adapted protocols for high-pressure freezing of cultivated cells from a physiological state. In this chapter, we describe optimized high-pressure freezing and freeze substitution protocols for STEM tomography in order to obtain high membrane contrast.

Ca/P concentration ratio at different sites of normal and osteoporotic rabbit bones evaluated by Auger and energy dispersive X-ray spectroscopy

Osteoporosis is a systemic skeletal disorder associated with reduced bone mineral density and the consequent high risk of bone fractures. Current practice relates osteoporosis largely with absolute mass loss. The assessment of variations in chemical composition in terms of the main elements comprising the bone mineral and its effect on the bone's quality is usually neglected. In this study, we evaluate the ratio of the main elements of bone mineral, calcium (Ca), and phosphorus (P), as a suitable in vitro biomarker for induced osteoporosis. The Ca/P concentration ratio was measured at different sites of normal and osteoporotic rabbit bones using two spectroscopic techniques: Auger electron spectroscopy (AES) and energy-dispersive X-ray spectroscopy (EDX). Results showed that there is no significant difference between samples from different genders or among cortical bone sites. On the contrary, we found that the Ca/P ratio of trabecular bone sections is comparable to cortical sections with induced osteoporosis. Ca/P ratio values are positively related to induced bone loss; furthermore, a different degree of correlation between Ca and P in cortical and trabecular bone is evident. This study also discusses the applicability of AES and EDX to the semiquantitative measurements of bone mineral's main elements along with the critical experimental parameters.

Electron microscopy techniques to study bacterial adhesion

Since its introduction 70 years ago electron microscopy has become an invaluable tool for microbiology and the study of bacterial interaction. Technological development over the past decades has enabled researchers to resolve smaller and smaller details in bacterial samples, while new preparation techniques like cryo preparation now allow to investigate bacteria even closer to their natural state. In this chapter we give a brief overview of electron microscopy techniques suitable for the investigation of bacterial adhesion at molecular as well as cellular level and a short outlook on future technologies relevant to the field.

Low voltage high-resolution SEM (LVHRSEM) for biological structural and molecular analysis

High-resolution scanning electron microscopy (HRSEM) is being used increasingly to gain new insights into three-dimensional organization of biological structure, macromolecular complexes and interactions of cellular components as well as isolated cell organelles. Modern scanning electron microscopes (SEMs) combined with adequate sample preparation can now provide resolution comparable with that achieved using transmission electron microscopes (TEMs) down to 2-5 nm for biological material. The versatility of the instrument and new sample preparation techniques have allowed detailed analysis of chromosomes, cytoskeletal components, virus and other biological material that has not been possible with TEM. The present review addresses resolution and specific specimen preparations for HRSEM, and highlights the importance of specimen preparation and choice of methods to achieve optimal results for proteins, macromolecular complexes and subcellular structures using low voltage HRSEM (LVHRSEM).

Optical microscopy versus scanning electron microscopy in urolithiasis

Stone analysis is incompletely done in many clinical centers. Identification of the stone component is essential for deciding future prophylaxis. X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy (SEM) still remains a distant dream for routine hospital work. It is in this context that optical microscopy is suggested as an alternate procedure. The objective of this article was to assess the utility of an optical microscope which gives magnification of up to 40x and gives clear picture of the surface of the stones. In order to authenticate the morphological analysis of urinary stones, SEM and elemental distribution analysis were performed. A total of 250 urinary stones of different compositions were collected from stone clinic, photographed, observed under an optical microscope, and optical photographs were taken at different angles. Twenty-five representative samples among these were gold sputtered to make them conductive and were fed into the SEM machine. Photographs of the samples were taken at different angles at magnifications up to 4,000. Elemental distribution analysis (EDAX) was done to confirm the composition. The observations of the two studies were compared. The different appearances of the stones under optical illuminated microscopy were mostly standardized appearances, namely bosselations of pure whewellite, spiculations of weddellite, bright yellow colored appearance of uric acid, and dirty white amorphous appearance of phosphates. SEM and EDAX gave clearer pictures and gave added confirmation of the stone composition. From the references thus obtained, it was possible to confirm the composition by studying the optical microscopic pictures. Higher magnification capacity of the SEM and the EDAX patterns are useful to give reference support for performing optical microscopy work. After standardization, routine analysis can be performed with optical microscopy. The advantage of the optical microscope is that, it is easy to use and samples can be analyzed in natural color.

Elemental distribution analysis of urinary crystals

Various crystals are seen in human urine. Some of them, particularly calcium oxalate dihydrate, are seen normally. Pathological crystals indicate crystal formation initiating urinary stones. Unfortunately, many of the relevant crystals are not recognized in light microscopic analysis of the urinary deposit performed in most of the clinical laboratories. Many crystals are not clearly identifiable under the ordinary light microscopy. The objective of the present study was to perform scanning electron microscopic (SEM) assessment of various urinary deposits and confirm the identity by elemental distribution analysis (EDAX). 50 samples of urinary deposits were collected from urinary stone clinic. Deposits containing significant crystalluria (more than 10 per HPF) were collected under liquid paraffin in special containers and taken up for SEM studies. The deposited crystals were retrieved with appropriate Pasteur pipettes, and placed on micropore filter paper discs. The fluid was absorbed by thicker layers of filter paper underneath and discs were fixed to brass studs. They were then gold sputtered to 100 A and examined under SEM (Jeol JSM 35C microscope). When crystals were seen, their morphology was recorded by taking photographs at different angles. At appropriate magnification, EDAX probe was pointed to the crystals under study and the wave patterns analyzed. Components of the crystals were recognized by utilizing the data. All the samples analyzed contained significant number of crystals. All samples contained more than one type of crystal. The commonest crystals encountered included calcium oxalate monohydrate (whewellite 22%), calcium oxalate dihydrate (weddellite 32%), uric acid (10%), calcium phosphates, namely, apatite (4%), brushite (6%), struvite (6%) and octocalcium phosphate (2%). The morphological appearances of urinary crystals described were correlated with the wavelengths obtained through elemental distribution analysis. Various urinary crystals that are not reported under light microscopy could be recognized by SEM-EDAX combination. EDAX is a significant tool for recognizing unknown crystals not identified by ordinary light microscopy or SEM alone.

EDAX versus FTIR in mixed stones

Mixed stones form a significant number of all urinary stones. Accurate analysis of individual areas of stones is fraught with uncertainties. Scanning electron microscopy with elemental distribution analysis (SEM-EDAX) is a very important tool in assessing stone composition. The objective of this paper is to project the role of the combination of Fourier transform infrared (FTIR) spectroscopy and SEM-EDAX combination in achieving a total understanding of mixed stone morphology. Ten mixed urinary stones were washed and dried and the composition recognized by analysis of FTIR spectra by comparing with the spectra of pure components. Spectra for different layers were obtained. Then the stone samples were further studied by SEM-EDAX analysis. The findings of FTIR were correlated with SEM-EDAX and detailed data generated. Using SEM-EDAX, the spatial distribution of major and trace elements were studied to understand their initiation and formation. As much as 80% of the stones studied were mixtures of calcium oxalate monohydrate (whewellite) and calcium phosphate (hydroxyapatite) in various proportions. Quantitative evaluation of components was achieved through FTIR and SEM-EDAX analysis. It was possible to get an idea about the spatial distribution of molecules using SEM analysis. The composition of different areas was identified using EDAX. Analyzing with EDAX, it was possible to obtain the percentage of different elements present in a single sample. The study concludes that the most common mixed stone encountered in the study is a mixture of calcium oxalate monohydrate and calcium phosphate in a definite proportion. The combination identified not only the molecular species present in the calculus, but also the crystalline forms within chemical constituents. Using EDAX, the amount of calcium, phosphorus, oxygen and carbon present in the stone sample could be well understood.

Electron microscopy of viruses and virus-cell interactions

Electron microscopy is a powerful tool to visualize viruses in diagnostic as well as in research settings for investigating viral structure and virus-cell interactions. Here, a simple but efficient method is described for demonstrating viruses by negative staining, and its limit is discussed. A prerequisite to obtain reliable information on virus-cell interactions is excellent preservation of cellular and viral ultrastructure. The crux is that during fixation and embedding, by applying conventional protocols about 50% of the lipids are lost, which results in loss of integrity of cell membranes. To achieve good preservation of cellular architectures, good contrast, and both high spatial and temporal resolution, methods for freezing, freeze-substitution, and freeze-etching are described and their applicability discussed mostly taking complicated built herpes viruses as examples.

Penetration and growth of DPPC/DHPC bicelles inside the stratum corneum of the skin

The effect of dipalmitoyl phosphatidylcholine (DPPC)/dihexanoyl phosphatidylcholine (DHPC) bicelles on the microstructure of pig stratum corneum (SC) in vitro was evaluated. The physicochemical characterization of these nanoaggregates revealed small disks with diameters around 15 nm and a thickness of 5.4 nm. Upon dilution, the bicelles grow and transform into vesicles. Cryogenic scanning electron microscopy (cryo-SEM) images of the SC pieces treated with this system showed vesicles of about 200 nm and lamellar-like structures in the intercellular lipid areas. These vesicles probably resulted from the growth and molecular rearrangement of the DPPC/DHPC bicelles after penetrating the SC. The presence of lamellar-like structures is ascribed to the interaction of the lipids from bicelles with the SC lipids. The bicellar system used is suitable to penetrate the skin SC and to reinforce the intercellular lipid areas, constituting a promising tool for skin applications.

Pros and cons: cryo-electron microscopic evaluation of block faces versus cryo-sections from frozen-hydrated skin specimens prepared by different techniques

Over the last two decades, several different preparative techniques have been developed to investigate frozen-hydrated biological samples by electron microscopy. In this article, we describe an alternative approach that allows either ultrastructural investigations of frozen human skin at a resolution better than 15 nm or sample throughput that is sufficiently high enough for quantitative morphological analysis. The specimen preparation method we describe is fast, reproducible, does not require much user experience or elaborate equipment. We compare high-pressure freezing with plunge freezing, and block faces with frozen-hydrated slices (sections), to study variations in cell thickness upon hydration changes. Plunge freezing is optimal for morphological and stereological investigations of structures with low water content. By contrast, high-pressure freezing proved optimal for high-resolution studies and provided the best ultrastructural preservation. A combination of these fast-freezing techniques with cryo-ultramicrotomy yielded well-preserved block faces of the original biological material. Here we show that these block faces did not exhibit any of the artefacts normally associated with cryo-sections, and--after evaporating a heavy metal and carbon onto the surface--are stable enough in the electron beam to provide high-resolution images of large surface areas for statistical analysis in a cryo-SEM (scanning electron microscope). Because the individual preparation steps use only standard equipment and do not require much experience from the experimenter, they are generally more usable, making this approach an interesting alternative to other methods for the ultrastructural investigation of frozen-hydrated material.

Impairment of nuclear pores in bovine herpesvirus 1-infected MDBK cells

Herpesvirus capsids originating in the nucleus overcome the nucleocytoplasmic barrier by budding at the inner nuclear membrane. The fate of the resulting virions is still under debate. The fact that capsids approach Golgi membranes from the cytoplasmic side led to the theory of fusion between the viral envelope and the outer nuclear membrane, resulting in the release of capsids into the cytoplasm. We recently discovered a continuum from the perinuclear space to the Golgi complex implying (i) intracisternal viral transportation from the perinuclear space directly into Golgi cisternae and (ii) the existence of an alternative pathway of capsids from the nucleus to the cytoplasm. Here, we analyzed the nuclear surface by high-resolution microscopy. Confocal microscopy of MDBK cells infected with recombinant bovine herpesvirus 1 expressing green fluorescent protein fused to VP26 (a minor capsid protein) revealed distortions of the nuclear surface in the course of viral multiplication. High-resolution scanning and transmission electron microscopy proved the distortions to be related to enlargement of nuclear pores through which nuclear content including capsids protrudes into the cytoplasm, suggesting that capsids use impaired nuclear pores as gateways to gain access to the cytoplasmic matrix. Close examination of Golgi membranes, rough endoplasmic reticulum, and outer nuclear membrane yielded capsid-membrane interaction of high identity to the budding process at the inner nuclear membrane. These observations signify the ability of capsids to induce budding at any cell membrane, provided the fusion machinery is present and/or budding is not suppressed by viral proteins.

Influence of chemical and freezing fixation methods in the freeze-fracture of stratum corneum

A comparison between two fixation techniques for freeze-fracture was established. Stratum corneum (SC) samples from pig epidermis were fixed using high-pressure freezing (HPF) and using plunging in propane freezing; the latter after chemical fixation. Then, frozen samples were freeze-fractured, coated with platinum-carbon, and visualized using a high-resolution low-temperature scanning electron microscope and a transmission electron microscope. Our results indicate that the plane of freeze-fracture was different depending on the fixation and freezing methodology used. In the samples frozen by HPF without chemical fixation, the fracture plane laid mainly between the lipid lamellae. However, when chemical fixation and plunging in propane freezing was used, the fracture plane did not show preference to a specific way. Plunging in propane freezing of chemically fixed samples, on the other hand, provides a more homogeneous fracture behaviour. Thus, depending on the methodology used, we can favour a visualization of either lipid or protein domains of the SC. These results could be very useful in future ultrastructural studies in order to facilitate the microscopic visualization and interpretation of the complex images such as those of SC and even of other samples in which different domains coexist.

Dead but Highly Dynamic – The Stratum corneum Is Divided into Three Hydration Zones

Topically applied water exerts mechanical stress on individual corneocytes as well as on the whole stratum corneum (SC), resulting in an alteration of barrier function. In this study we used complete skin biopsies and showed that the SC reacts to water stress as a highly optimized and well-regulated structure against osmotic changes. Following a relatively new cryo-processing protocol for cryo-SEM, it is possible to reliably maintain and investigate the hydrated state of the SC and individual corneocytes after treatment with solutions of different ionic strength. Treatment with distilled water results in swelling of SC cells together with formation of massive water inclusions between adjacent cell layers. Treatment with 5-20% NaCl reveals three different hydration zones within the SC: Corneocytes near the live-dead transition zone can swell to nearly double their thickness. The second zone is the most compact, as the corneocytes here show the smallest thickness variation with all treatments. Within the outermost zone, again a massive swelling and loosening of intracellular filament packing can be observed. We therefore conclude that the SC itself is subdivided into three functional zones with individual water penetration and binding potentials. Since the second zone remains nearly unaffected by water stress, we propose that this zone hosts the functional SC barrier.

Method of platinum-carbon coating of ultrathin sections for transmission and scanning electron microscopy: An application for study of biological composites

Many biological materials are composites containing two or more components with different mechanical properties. This study is concerned with the application of a method of platinum-carbon coating (Pt/C) of ultrathin sections for TEM and SEM studies of the design of natural composite materials. The changes in profile of the ultrathin resin-embedded sections during different stages of the preparation reflect the material properties of the various components: stiffer regions deform less than softer ones. Such changes in the section profile can be visualized by the Pt/C method and used as evidence of specific material properties in particular regions of composite materials. The method increases the relief contrast, improves the 3D-view of structures, and in combination with standard TEM and SEM procedures can provide clear demonstrations of material design. The distribution of chitin crystallites in the insect cuticle and the ultrastructure of the pore canal system specialized for the transport of epidermal secretions to the cuticle surface were studied here as examples.

High-resolution CryoFESEM of individual cell adhesion molecules (CAMs) in the glycocalyx of human platelets: detection of P-selectin (CD62P), GPI-IX complex (CD42A/CD42B alpha,B beta), and integrin GPIIbIIIa (CD41/CD61) by immunogold labeling and stereo imaging

The aim of this study was to develop a model for the detection of individual cell adhesion molecules (CAMs) in the glycocalyx of spread human platelets using high-resolution cryo-field emission scanning electron microscopy (cryoFESEM). Three surface glycoprotein CAMs, P-selectin (CD62P), GPIba in the GPI-IX complex (CD42a/CD42b alpha,b beta), and the integrin GPIIbIIIa (CD41/CD61) in the human platelet were selected on the basis of their unique topographic shape. Spread human platelets were indirectly immunolabeled with 10-nm colloidal gold and then cryoimmobilized. After sublimation of water from the cryoimmobilized sample, partially freeze-dried platelets were coated unidirectionally with Pt, stabilized with carbon, and examined in an in-lens cryoFESEM using high-resolution backscattered electron imaging. CAMs were detected by indirect immunogold labeling and the length of each type of CAM was determined using analysis of differences in parallax as measured in the software program Sterecon. Our results demonstrate the efficacy of using high-resolution cryoFESEM to recognize and detect individual CAMs in the glycocalyx. Further advances in production of metal coatings with finer granularity, together with improvements in imaging (tilting and angle of stereo images), may provide better definition of the topography associated with glycosylation and formation of multimeric CAM complexes. (J Histochem Cytochem 49:809-819, 2001)

Liposomes as protective agents of stratum corneum against octyl glucoside: a study based on high-resolution, low-temperature scanning electron microscopy

The ability of phosphatidylcholine (PC) liposomes to protect pig stratum corneum (SC) against the action of the nonionic surfactant octyl glucoside (OG) was investigated "in vitro" using double-layer coating for high-resolution, low-temperature scanning electron microscopy. This technique has been useful in preventing drying artifacts in the study of biological materials. The treatment of SC with OG led to a perturbation mainly in the corneocytes. However, the incubation of the tissue with liposomes prior to the OG treatment resulted in a progressive decrease in these perturbations and, consequently, in the progressive protection of the SC against the action of the surfactant.

Electron microscopic investigation of water occlusions in intercellular spaces in the inner cortex of lucerne nodules

It is unclear to what extent oxygen diffusion pathways through the cortex of the nitrogen-fixing zone of indeterminate nodules are liquid filled and whether a blockage of these pathways is involved in varying nodule oxygen permeability to control nitrogenase activity. We examined the proportion of water-filled intercellular spaces of lucerne (Medicago sativa L.) nodules with cryo-scanning electron microscopy. This technique allows for direct observation of water accumulation. Thirty percent of all intercellular spaces in the inner cortex of lucerne nodules were liquid filled. Decreasing the nodule oxygen permeability by detopping of the plant or by increasing the rhizospheric oxygen partial pressure to 80 kPa had no statistically significant effect on the water distribution in the intercellular spaces. Therefore, the hypothesis of a continuous aqueous diffusion barrier in the inner cortex could not be supported. The abundance of glycoproteins in intercellular spaces of the inner cortex was investigated with immunoelectron microscopy. No alteration due to detopping or after increase of the rhizospheric oxygen partial pressure was observed. Therefore, our results do not support the hypothesis of a short-term regulation of oxygen permeability by blockage of diffusion pathways through morphological changes in the cortex region of the nitrogen-fixing zone of lucerne nodules.

Asexual sporulation in coprinus cinereus: structure and development of oidiophores and oidia in an amut bmut homokaryon

Polak, E., Hermann, R., Kues, U., and Aebi, M. 1997. Asexual sporulation in Coprinus cinereus: Structure and development of oidiophores and oidia in an Amut Bmut homokaryon. 22, 112-126. Coprinus cinereus strain AmutBmut is a homokaryon with mutations in both mating type loci. It produces asexual spores (oidia) in sticky liquid droplets on specialized aerial structures (oidiophores). These oidiophores have uninucleate cells and are organized as those of the monokaryon 5026 from which the strain derived. However, unlike in the monokaryon, oidiophores in strain AmutBmut are induced by light. Young oidiophores are easily detected upon light induction and the process of oidiophore development is readily followed in this strain. Fully grown oidiophores consecutively give rise to short branches (oidial hyphae) that break up into two or occasionally three uninucleate oidia (arthroconidia) until up to 200 oidia are collected at the tip of the oidiophore. Mature spores are enclosed by a mucilage and a double-layered primary cell wall with hair-like structures except for the sides of former cell attachments. In a summary of our microscopic observations on developing oidiophores and nuclear stainings we present a model showing the successive steps of oidiophore and spore development.

Double-layer coating for field-emission cryo-scanning electron microscopy--present state and applications

Imaging of fast-frozen samples is the most direct approach for electron microscopy of organic material. It prevents chemical fixation and drying artifacts. Frozen samples can be replicated and imaged in the transmission electron microscope (TEM), or they can be directly visualized in the cryo-scanning electron microscope (cryo-SEM). Double-layer coating combines these two techniques and many of their advantages. With this method, the frozen bulk sample is coated similar to the TEM-replica technique with, for example, a shadow of platinum (at an angle of 45 degrees) and an additional layer of carbon. Then, the sample is cryo-transferred to an SEM equipped with a cold stage and imaged with the material-dependent backscattered electron signal that shows the platinum distribution. With this method, charging artifacts and the effects of beam damage are significantly reduced. Although currently the resolution of the replica technique cannot be surpassed, the method greatly facilitates the processing of brittle, rapidly frozen samples because no replica cleaning is necessary. This makes the method especially suitable for high-pressure frozen samples.